Field
The present disclosure relates to the downward flow of water in the downcomer region of a natural circulation type boiling water nuclear reactor.
Description of Related Art
FIG. 1 is a cross-sectional view of a conventional natural circulation type boiling water nuclear reactor. Referring to FIG. 1, a conventional natural circulation type boiling water nuclear reactor 100 (e.g., economic simplified boiling water reactor (ESBWR)) uses a relatively long chimney 108 between the outlet of the reactor core 112 and the steam separator inlet to establish, enhance, and deliver natural circulation steam up the center region and water down flow 104 in the downcomer region 106 of the reactor pressure vessel 102. In particular, feedwater coolant enters the reactor pressure vessel 102 and mixes with the cycling liquid water separated from the steam and by the force of gravity starts a downward flow 104 in the downcomer region 106. Some mixing occurs from the water exiting the steam separators and steam dryers. The downcomer region 106 is the space between the outer wall of the chimney 108 and the inner wall of the reactor pressure vessel 102. The chimney 108 has a geometry of a right cylinder body of revolution having a constant radius that is about 8.6 meters in length within the reactor pressure vessel 102, which is about 27.6 meters. It should be understood that this dimension is merely provided to give scale rather than to define an absolute length. Next, the water flows toward the outside of the core 112 and returns flow to the core 112 by making a complete flow reversal in the core inlet region 114. The water flows vertically upward due to the decreasing density above the core 112 as a result of steam formation and the water flow in the downcomer region 106. The core of heat-producing fissionable fuel is located above the core inlet region 114.
Conventionally, there are no flow disruptions as the water flows down the outside of the chimney 108. The chimney 108 has a flow length of approximately 8.6 meters and is one of the largest internal components in the nuclear reactor 100. While the chimney 108 has internal partitions to ensure the steam water mixture flows in the vertical direction, there are no provisions to mix the water flow 104 on the outside of the chimney 108. There are also insignificant frictional losses within the downcomer region 106. Furthermore, there is neither an increase in the enthalpy of the steam water mixture inside the chimney 108 nor is there any enthalpy change in the water flowing in the downcomer region 106.
FIG. 2 is a cross-sectional view of a conventional chimney in a natural circulation type boiling water nuclear reactor. Referring to FIG. 2, the exterior surface 110 of the chimney 108 is relatively smooth. There is a lack of homogeneous temperature of the water in the downcomer region 106 due to the relative smoothness of the downcomer region 106. The 8.6 meter height of the chimney 108 results in a flow 104 in the downcomer region 106 to be very similar to a water fall such that there is an increasing downward velocity gradient with relatively little mixing in the axial or radial directions. The lack of a uniform temperature of the water results in non-uniform power generation in the reactor core 112.